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Lake Superior water warms faster than usual

The summer of 2010 is shaping up as one with some of the warmest water temperatures ever for the world’s largest lake, according to researchers at the Large Lakes Observatory (LLO) of the University of Minnesota Duluth.

“As always, changing water temperatures in Lake Superior have a wide range of implications, from the productivity of microbes and algae in the lake to swimming temperature at tourist beaches,” LLO said in a news statement.

Each year, after the winter ice melts, the surface waters begin to warm up. When they reach about 39 degrees F, all of the water in the lake is at the same temperature and density, and the wind causes the lake to “turn over” or mix thoroughly, LLO explained.

“It did so unusually early this year, in early- to mid-June rather than the normal mid-July. From now until the fall, the surface waters warm and stay on top, so that the water column is stratified.”

When LLO researcher Jay Austin noticed that the lake was headed for an early turnover this year, he said that this “typically means that it’s going to be a very warm year in Lake Superior.”

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In 2007 Austin and fellow researcher Steve Colman showed that summer water temperatures were warming twice as fast as air temperatures over the last 30 years, based on data from NOAA buoys in Lake Superior. “The reason for the warming was partly due to increasing air temperatures, but also strikingly related to winter ice cover,” LLO said. Steve Colman is also a member of the National Geographic Society’s Committee for Research and Exploration.

The less winter ice, the earlier the spring turnover, and the longer the summer heating season for the surface waters, according to LLO.

“Watching last winter’s ice cover, Austin says he suspected that we were in for warm summer water temperatures as much as five months ago. The early turnover date this year confirms his suspicions.”

Warmer lake leads to stronger winds

Rising water temperatures are kicking up more powerful winds on Lake Superior, with consequences for currents, biological cycles, pollution and more on Lake Superior, scientists from the University of Minnesota-Duluth reported last November.

A wide temperature differential between water and air makes for a more stable atmosphere with calmer winds over the relatively cold water. However, as warming water closes the gap, as in Lake Superior’s case, the atmosphere gets more turbulent, said Ankur Desai, atmospheric and oceanic sciences professor at the University of Wisconsin-Madison.

“You get more powerful winds,” Desai said in a news statement at the time. “We’ve seen a 5 percent increase per decade in average wind speed since 1985.”

The findings were published in the journal Nature Geoscience.

Lake Superior may be the anchor for a chain of lakes that hold one-fifth of the world’s fresh surface water, but the impact of steadily rising temperatures has been poorly understood, Desai said.